Identify the monomer that is repeated in the polymer chain. For addition polymers, take out one of the bonds in the double bond and stick it on the end of the monomer (either side) and this will form a covalent bond with the end of another monomer. For condensation polymers, identify the functional on the end of the monomer (usually carboxylic acid group COOH or alcohol group OH), get two of these monomers with these functional groups and remove a H2O, ie take a H from one monomer and an OH from another. This leaves your two monomers which covalently bond to form the start of your long polymer chain. Hope that helps.